Files that are deleted from the hard drive can still be recovered by using an application that is often referred to as data recovery software or other similar terms. How could that happen?

In Windows, if a file is deleted by pressing the Delete key on the keypad aka keyboard, then it will just put that file in the Recycle Bin folder. That file can be restored easyly by pressing restore button on the Recycle Bin. But, if we pressing Shift + Delete key, Windows will permanently delete that files or by clicking emptying the Recycle Bin, anyway that file can also be restored too. File from a formatted hard drive can be restored too. Why?

Operating system has a File Allocation Table (FAT) used to build the directory structure as seen in Windows Explorer in the Windows operating system. Files that are stored on disk is scattered in various sectors. FAT contains a pointer to indicate the position of the file on the hard drive. So, despite the neat look in Explorer, not the case in the hard disk.

At the time computer users delete some files, actually its just a pointer of that file are deleted. File still residing in the disk, but the operating system no longer detect it. Because the deleted files were actually still exist in hard drive, with the help of recovery software, its easy to restore it. Recovery software detects the files that do not have a pointer and display it. By the time a file is restored, recovery software will create a pointer in the FAT's operating system, so the operating system is able to detect that file again.

Not guaranteed to be 100% of a large file that could come back again. Since it is very likely spread across different sectors so that it has a lot of pointers. By the time he was removed, the pointer was also removed. When the file is returned, it does not return results as expected. The file is incomplete. What happened? Is it because the recovery software that we uses not good? It might, but probably most of it because the hard drive is filled by a new file. By the time a new file is inserted into the disk, the operating system will place a file on sectors that are considered empty. It could be that the real sector contains a large part of the file that had been returned. Well, the part that was buried for a long time could not be returned back!

Software Reference: Recover Deleted Files